Fix Zsh completion for tasks without description

[carlsmedstad]

Use --list-all instead of --list in order include tasks without
description in the auto-completion.

[andreynering]

Thanks @carlsmedstad!

[ghostsquad]

This was so needed. In fact, I'd like to open another pull request.. I feel like --list should include all tasks regardless, and you would just add another option to filter to include only those with descriptions. E.g.

# old command
task --list

# new
task --list --only-described

# and deprecate this
task --list-all

[ghostsquad]

I suppose it's important to add a description to tasks that a user may want to run...

Nvm, that way --list becomes the easiest thing to get the tasks that are most relevant.

I'll open an issue with this once I have some more time to think on it.